The present invention generally relates to constructing subsurface structures by injecting a fracture fluid to fracture underground formations, and more particularly to a method and apparatus for creating a vertical fracture oriented at a predetermined azimuth in unconsolidated and weakly cemented soils and sediments resulting in subsurface containment walls and or treatment barriers.
Subsurface structures are installed to perform various environmental, geotechnical, and petroleum recovery functions. In the case of environmental and geotechnical applications, containment walls and treatment barriers are typically installed to extend from the ground surface to a subsurface zone. In these applications, the containment walls may include flow containment walls which contain the flow of underground liquids, and treatment barriers which are permeable zones filled with reactive material. In many cases, the construction process must penetrate many feet below the ground surface before reaching a subsurface zone that requires a structure such as a containment wall or a treatment barrier. Examples of construction techniques performed in the prior art are sheet piling walls, slurry walls, braced excavations, and continuous trenches.
Current construction techniques required to install the above containment walls and treatment barriers share many common problems, such as the necessity to reroute underground utilities, potential structural damage to existing buildings and structures, potentially large staging areas for construction equipment, specialized and expensive equipment. In many cases, the removal and proper disposal of contaminated soils and liquids recovered from the excavation is required. Most of the above examples, for either economical or technical reasons, have a maximum wall or barrier depth that may not allow a project to be completed or even begun.
Turning now to the prior art, hydraulic fracturing of subsurface earth formations to stimulate production of hydrocarbon fluids from subterranean formations has been carried out in many parts of the world for over fifty years. The earth is hydraulically fractured either through perforations in a cased well bore or in an isolated section of an open bore hole. The horizontal and vertical orientation of the hydraulic fracture is controlled by the compressive stress regime in the earth and the fabric of the formation. It is well known in the art of rock mechanics that a fracture will occur in a plane perpendicular to the direction of the minimum stress, see U.S. Pat. No. 4,271,696 to Wood. At significant depth, one of the horizontal stresses is generally at a minimum, resulting in a vertical fracture formed by the hydraulic fracturing process. It is also well known in the art that the azimuth of the vertical fracture is controlled by the orientation of the minimum horizontal stress.
At shallow depths, the horizontal stresses could be less or greater than the vertical overburden stress. If the horizontal stresses are less than the vertical overburden stress, then vertical fractures will be produced; whereas if the horizontal stresses are greater than the vertical overburden stress, then a horizontal fracture will be formed by the hydraulic fracturing process.
Techniques to induce a preferred horizontal orientation of the fracture from a well bore are well known. These techniques include slotting, by either a gaseous or fluid jet under pressure, to form a horizontal notch in an open bore hole. Such techniques are commonly used in the petroleum and environmental industry. The slotting technique performs satisfactorily in producing a horizontal fracture, provided that the horizontal stresses are greater than the vertical overburden stress, or the earth formation has sufficient horizontal layering or fabric to ensure that the fracture continues propagating in the horizontal plane. Perforations in a horizontal plane to induce a horizontal fracture from a cased well bore have been disclosed, but such perforations do not preferentially induce horizontal fractures in formations of low horizontal stress. See U.S. Pat. No. 5,002,431 to Heymans.
Various means for creating vertical slots in a cased well bore have been disclosed. The prior art recognizes that a chain saw can be used for slotting the casing. See U.S. Pat. No. 1,789,993 to Switzer; U.S. Pat. No. 2,178,554 to Bowie, et al., U.S. Pat. No. 3,225,828 to Wisenbaker; and U.S. Pat. No. 4,119,151 to Smith. Installing pre-slotted or weakened casing has also been disclosed in the prior art as an alternative to perforating the casing. See U.S. Pat. No. 5,103,911 to Heijnen. These methods in the prior art were not concerned with the azimuth orientation of two opposing slots for the preferential initiating of a vertical hydraulic fracture at a predetermined azimuth orientation. It has been generally accepted in the art that the fracture azimuth orientation cannot be controlled by such means. These methods were an alternative to perforating the casing to achieve better connection between the well bore and the surrounding formation.
In the art of hydraulic fracturing subsurface earth formations from subterranean wells at depth, it is well known that the earth""s compressive stresses at the region of fluid injection into the formation will typically result in the creation of a vertical two xe2x80x9cwingedxe2x80x9d structure. This xe2x80x9cwingedxe2x80x9d structure generally extends laterally from the well bore in opposite directions and in a plane generally normal to the minimum in situ horizontal compressive stress. This type of fracture is well known in the petroleum industry as that which occurs when a pressurized fracture fluid, usually a mixture of water and a gelling agent together with certain proppant material, is injected into the formation from a well bore which is either cased or uncased. Such fractures extend radially as well as vertically until the fracture encounters a zone or layer of earth material which is at a higher compressive stress or is significantly strong to inhibit further fracture propagation without increased injection pressure.
It is also well known in the prior art that the azimuth of the vertical hydraulic fracture is controlled by the stress regime with the azimuth of the vertical hydraulic fracture being perpendicular to the minimum horizontal stress direction. Attempts to initiate and propagate a vertical hydraulic fracture at a preferred azimuth orientation have not been successful, and it is widely believed that the azimuth of a vertical hydraulic fracture can only be varied by changes in the earth""s stress regime. Such alteration of the earth""s local stress regime has been observed in petroleum reservoirs subject to significant injection pressure and during the withdrawal of fluids resulting in local azimuth changes of vertical hydraulic fractures.
The determination of the hydraulic fracture geometry, such as its horizontal or vertical orientation, azimuth and length of the vertical fracture, and the extent and depth of a horizontal fracture, can be made from the measurement of earth tilts by conventional surface or bore hole mounted biaxial tiltmeters, see U.S. Pat. No. 4,271,696 to Wood. Highly sensitive electronic tiltmeters, capable of measuring tilts less than 10xe2x88x927 radians, measure the earth""s deformation due to the opening and propagation of a hydraulic fracture. From monitoring these tilts in real time along with the flow of injected fluid, the hydraulic fracture geometry can be determined. See U.S. Pat. Nos. 4,271,696 and 4,353,244 to Wood. Influence functions that relate the opening of a fracture to ground deformation can be utilized to calculate the fracture geometry. As suggested by U.S. Pat. No. 5,002,431 to Heymans, the fracture geometry can be determined and controlled from the measurement of tilts and real time computer control. Heymans does not detail how the fracture geometry may be determined, nor does Heymans disclose how the interaction of ground tilts from multiple fractures can be resolved to determine fracture geometry.
The method of determining the hydraulic fracture geometry, disclosed by U.S. Pat. No. 4,353,244 to Wood, has a number of deficiencies. If (a) the fracture is non-planar, (b) if the fracture is not of the full initiated height, or (c) if multiple fractures are initiated in close proximity of each other, then fracture geometry determination is not assured.
Accordingly, there is a need for a method and apparatus for controlling the azimuth orientation of a vertical hydraulic fracture in formations of unconsolidated or weakly cemented sediments and soils.
There is a further need for a method and apparatus for monitoring and calculating in real time the propagation of the azimuth of vertical hydraulic fractures.
And, there is a further need for a method and apparatus for the creation and control of coalesced, overlapping, and interconnecting fractures to form a treatment barrier or containment wall formed from a fracture fluid.
The present invention is a method and apparatus for dilating the earth by various means from a bore hole or driven lance to initiate and to control the azimuth orientation of a vertical hydraulic fracture in formations of unconsolidated or weakly cemented sediments. The fracture is initiated by means of preferentially dilating the earth orthogonal to the desired azimuth direction. This dilation of the earth can be generated by a variety of means: a driven spade to dilate the ground orthogonal to the required azimuth direction, packers that inflate and preferentially dilate the ground orthogonal to the required azimuth direction, pressurization of a pre-weakened casing with the weaknesses aligned in the required azimuth orientation, pressurization of a casing with opposing slots cut along the required azimuth direction, or pressurization of a two xe2x80x9cwingedxe2x80x9d artificial vertical fracture generated by cutting or slotting the casing, grout, and/or formation at the required azimuth orientation.
In addition to initiation of vertical fractures at a predetermined azimuth, the present invention further provides a method and apparatus for monitoring and controlling the propagation of the fractures along the predetermined azimuth. The present invention relies upon the determination of influence functions that relate the earth""s deformation to the opening of an elementary fracture.
Using generally accepted principles of elasticity theory, influence functions such as Green""s functions can be applied to the problem. See xe2x80x9cHydrodynamicsxe2x80x9d by H. Lamb, 4th edition, Cambridge (1916) and xe2x80x9cTreatise on Mathematical Theories of Elasticityxe2x80x9d by A. E. H. Love, 4th edition, Dover Publications (1944). If a series of Green""s functions can be solved relating incremental measurements of the earth""s induced voltage or deformation with the geometry change in an initiated fracture, then a sequential incremental solution can be determined. With a series of sequential incremental solutions, an inverse model can be created for a particular type of soil and sediment formation or for a specific formation. The geometry of the fracture can then be calculated during the injection process by a series of sequential incremental solution calculations involving the inverse model. Utilizing either measured ground tilts or measured induced voltages from electrifying the fracturing fluid, the user monitors the injected flow of the injected fluid to determine and control the in situ geometry of the fracture during the injection process.
Thus, by relating measured incremental ground tilts or measured induced voltages to yield incremental change of the fracture geometry constrained by the incremental volume of injected fluid into each fracture, the incremental change in geometry of each fracture is found by minimizing the differences in the computed and measured incremental tilts or induced voltages. The essential difference between the present invention and the prior art is the utilization of a sequential incremental solution to determine incremental fracture geometry changes as the fracture is propagating. Such a method yields the fracture geometry at a particular time during the injection process. Without a sequential incremental solution method to determine the in situ fracture geometry, the system of equations is poorly defined, and a large range of differing geometry of fractures can fit or yield the same tilt field.
The present method and apparatus determine the fracture geometry by active resistivity after initiating a hydraulic fracture in moderate to highly resistive ground conditions. The fracture fluid is electrically conductive and is electrically energized by an alternating electrical source, typically a 100 Hz low voltage source. Real time instrumentation monitors resistivity receivers for surface and sub-surface induced voltages of the 100 Hz signal due to the energized fracture fluid in the propagating fracture. Surface and sub-surface induced voltages are recorded. By utilizing potential influence functions of the induced voltage in the earth due to an elementary electrified fracture, and from an incremental inverse model constrained by the incremental volume of injected fluid, the fracture geometry is determined in real time during the injection process.
The active resistivity method of monitoring fracture geometry requires that the fracture fluid be at least twenty times more conductive than the surrounding ground, to ensure a sharp signal and high contrast between the fracture and the surrounding medium. In this case, the energized fracture can be approximated as an electrified sheet, and potential influence functions of the earth""s induced voltages from an elementary electrified fracture can be formulated. The fracture fluid can be made conductive by the addition of soluble salts or by the selection of a suitable fracture fluid that has a high conductivity. Fracture fluids suitable for this method and apparatus can comprise, but are not limited to, a water based guar gum gel for a permeable treatment barrier and a bentonite cement slurry for an impermeable containment barrier.
From real time monitoring of the earth""s induced voltages due to the propagating electrified fracture and from the flow of injected fluid into the fracture, the geometry of the fracture can be determined. Influence functions relate the earth""s induced voltage to the propagation of an elementary electrified fracture. By solving the inverse problem of measured incremental induced voltages to yield fracture incremental geometry change constrained by the incremental volume of injected fluid into each fracture, the incremental change in geometry of each fracture can be found by minimizing the differences in the computed and measured incremental induced voltages.
If the initiated hydraulic fracture is relatively deep, down hole resistivity receivers are used to obtain a high precision image of the energized fracture. Active resistivity monitoring has the added benefit of determining when individual fractures coalesce and thus become electrically connected. That is, by energizing the fracture fluid in each injected well bore individually and in unison, the electrical coalescence of multiple fractures from different well bores can be clearly recorded and observed. The imaging and observation of the down hole resistivity data focuses on quantifying the continuity of the fractures and assessing the fracture continuity to determine if any holes or gaps are present. Such monitoring allows construction procedures to be modified to ensure the hydraulic fractures are installed as planned and allows contingency measures to be implemented immediately, e.g. an additional fracture to patch any hole or additional injection volumes to ensure coalescence or sufficient overlap.
The present invention also pertains to a method for constructing subsurface structures including containment walls or treatment barriers by injecting a liquid slurry into an azimuth controlled hydraulic fracture or fractures to form either a continuous or overlapping system of fractures. By initiating and propagating azimuth orientated vertical hydraulic fractures from a series of bore holes aligned in the required azimuth direction, coalesced and overlapping fractures are created to form a containment wall or treatment barrier composed of the fracture fluid. The process of monitoring and calculating the fracture in situ geometry during the injection process, enables determination of when to cease injection or whether to continue injection to achieve the required shape, extent, coalescence, or degree of overlap of the azimuth orientated vertical hydraulic fractures.
The fracture fluid used to form the containment walls and treatment barriers in the vertical fractures has two purposes. First the fracture fluid must be formulated in order to initiate and propagate the fracture within the underground formation. In that regard the fracture fluid has certain attributes. The fracture fluid fracture fluid should not leak off into the formation, the fracture fluid should be clean breaking with minimal residue, and the fracture fluid should have a low friction coefficient.
Second, once injected into the fracture, the fracture fluid forms the containment wall or the treatment barrier. In that regard, the fracture fluid comprises a proppant which produces the integrity for a containment wall or the active component for a treatment barrier. Such proppants for containment walls may include, for example, perlite in a bentonite cement slurry. Such proppants for treatment barriers may include, for example, iron filings. The proppants are selected and formulated to accomplish the purpose intended for the containment wall or the treatment barrier.
The present invention is applicable only to formations of unconsolidated or weakly cemented sediments and soils with low cohesive strength compared to the vertical overburden stress prevailing at the depth of the hydraulic fracture. Low cohesive strength is defined herein as the greater of 200 pounds per square inch (psi) or 25% of the total vertical overburden stress. The method is not applicable to consolidated rock formations, in which the fracture azimuth is controlled by the rock formation stress regime.
Although the present invention contemplates the formation of fractures which generally extend laterally away from a vertical or near vertical well penetrating an earth formation and in a generally vertical plane in opposite directions from the well, i.e. a vertical two winged fracture, those skilled in the art will recognize that the invention may be carried out in earth formations wherein the fractures and the well bores can extend in directions other than vertical.
Therefore, the present invention provides a method and apparatus for controlling the azimuth of a vertical hydraulic fracture in formations of unconsolidated or weakly cemented sediments and soils.
The present invention also provides a method and apparatus for the creation and control of coalesced, overlapping, and interconnecting fractures to form a containment barrier or treatment wall.
Further, the present invention provides a method and apparatus for monitoring and calculating in real time the propagation of a vertical hydraulic fracture.
Other objects, features and advantages of the present invention will become apparent upon reviewing the following description of the preferred embodiments of the invention, when taken in conjunction with the drawings and the claims.